Harissa is the fiercely hot chilli paste of Morocco, zhug its slightly more aromatic Israeli equivalent. They are used in small amounts to lift dishes and the more you eat, the better you like the fire they engender. If you eat harissa or zhug injudiciously and burn yourself, then treat the pain with spoonfuls of yogurt. Sugar also helps - one reason why desserts in countries where a lot of chillies are eaten are so often tooth-achingly sweet.
Harissa keeps well in a jar in the refrigerator though, given its ferocity, it is surprisingly not impervious to bacterial contamination. This is perhaps caused by children licking their fingers and dipping them into the harissa for a treat when bored with chocolate bars.